Photopolymerizable composition, pressure-sensitive flame-retardant adhesive, and adhesive sheets

ABSTRACT

Adhesive sheets using an acrylic pressure-sensitive flame-retardant adhesive having excellent properties such as adhesive force and holding power at high temperature and also having a self fire-extinguishing property which instantaneously extinguishes fire, i.e., a flame-retardance, are provided. A photopolymerizable composition comprising a) 100 parts by weight of a monomer (or the oligomer thereof) comprising 70 to 100% by weight of a (meth)acrylic acid alkyl ester having on the average a carbon number of 2 to 14 in the alkyl group and 30 to 0% by weight of a monoethylenically unsaturated monomer copolymerizable with the ester, b) 0.02 to 5 parts by weight of a polyfunctional (meth)acrylate as a crosslinking agent, c) 0.01 to 5 parts by weight of a photopolymerization initiator, and d) 10 to 180 parts by weight of a flame retardant having a melting point of at least 60° C. is prepared, a pressure-sensitive flame-retardant adhesive is prepared from the photopolymerized product of the composition, and the adhesive is formed on one or both surfaces of a substrate.

TECHNICAL FIELD

The present invention relates to a pressure-sensitive flame-retardantadhesive required to have a sufficient adhesive force and also a selffire-extinguishing property for instantaneously extinguishing fire inthe fields of fixation of electronic parts, building materials,vehicles, aircraft, ships, etc.; adhesive sheets formed by forming thepressure-sensitive flame-retardant adhesive into, for example, a sheetform or a tape form; and a photopolymerizable composition for obtainingthe pressure-sensitive flame-retardant adhesive.

BACKGROUND ART

An acrylic pressure-sensitive adhesive has generally been prepared bysolution-polymerizing monomers mainly comprising a (meth)acrylic acidalkyl ester. However, from the restrictions on safety of organicsolvents and environmental health, a method of preparing thepressure-sensitive adhesive by emulsion-polymerizing orphoto-polymerizing the above-described monomers is recently widely used.

Such a pressure-sensitive adhesive is usually that an acrylic adhesivepolymer is crosslinked with a crosslinking agent to increase a cohesiveforce at high temperature, thereby imparting an excellent heatresistance. However, since many of the adhesive components areconstituted of combustible materials, there has been a difficulty thatthe pressure-sensitive adhesive cannot be used in fields of applicationrequiring a flame-retardance together with a heat resistance.

To overcome this difficulty, JP-A-1-223187 (the term “JP-A” as usedherein means an “unexamined published Japanese patent application”), forexample, discloses a pressure-sensitive flame-retardant adhesiveobtained by adding a flame-retardant or a flame-retardant assistant,such as a brominated material, chlorinated paraffin, aluminum hydroxide,antimony trioxide, etc., to a adhesive base polymer such as an acrylicpolymer, a synthetic rubber, a natural rubber, etc.

However, it is necessary to add a large amount of a flame-retardant inorder to exhibit a sufficient flame retardant effect by such aconventional pressure-sensitive flame-retardant adhesive. In this case,there is a problem that the heat resistance of the pressure-sensitiveadhesive is decreased. Further, if the degree of crosslinking of thebase polymer, such as a rubber, is increased in order to improve theheat resistance, the adhesive property thereof is decreased, so that itis necessary to add a large amount of a tackifier. In this case, thereis a problem that holding power of the adhesive at high temperature isdecreased in addition to increase of the cost.

In view of these circumstances, an object of the present invention is toprovide an acrylic pressure-sensitive flame-retardant adhesive which hasexcellent adhesive performances such as adhesive force or holding athigh temperature, and also has a self fire-extinguishing property forinstantaneously extinguishing fire, that is, a flame-retardance;adhesive sheets obtained by forming the pressure-sensitiveflame-retardant adhesive in, for example, a sheet form or a tape form;and a photopolymerizable composition for obtaining the above-describedpressure-sensitive flame-retardant adhesive.

DISCLOSURE OF THE INVENTION

As a result of various investigations to attain the above-describedobject, the present inventors have found that if a photopolymerizablecomposition comprising a (meth)acrylic acid alkyl ester as a maincomponent and a specific crosslinking agent added thereto together witha photopolymerization initiator, and a specific flame-retardant furthercompounded therewith is polymerized and cured by irradiation withultraviolet rays, a pressure-sensitive flame-retardant adhesive havingexcellent adhesive performances such as adhesive force and holding powerat high temperature and also a self fire-extinguishing property forinstantaneously extinguishing fire, that is, a flame-retardant property,and the adhesive sheets thereof are obtained, and have accomplished thepresent invention.

That is, the present invention relates to a photopolymerizablecomposition comprising following components a) to d);

a) 100 parts by weight of monomer(s) (or an oligomer thereof) comprising70 to 100% by weight of a (meth)acrylic acid alkyl ester having on theaverage a carbon number of 2 to 14 in the alkyl group, and 30 to 0% byweight of a monoethylenically unsaturated monomer,

b) 0.02 to 5 parts by weight of a polyfunctional (meth)acrylate as acrosslinking agent,

c) 0.01 to 5 parts by weight of a photopolymerization initiator, and

d) 10 to 180 parts by weight of a flame-retardant having a melting pointof at least 60° C.;

a pressure-sensitive frame-retardant adhesive comprising thephotopolymerization product of this composition; and adhesive sheetshaving the layer of the pressure-sensitive flame-retardant adhesive onone surface or both surfaces of a substrate.

In the monomer of the component a) used in the present invention, the(meth)acrylic acid alkyl ester having on the average a carbon number of2 to 14 in the alkyl group includes ethyl (meth)acrylate, butyl(meth)acrylate, isoamyl (meth)acrylate, n-hexyl (meth)acrylate,2-ethylhexyl (meth)-acrylate, iso-octyl (meth)acrylate, isononyl(meth)acrylate, decyl (meth)acrylate, dodecyl (meth)acrylate, and thelike. Those can be used alone or as a mixture of two or more kinds ofthem.

If necessary, together with the (meth)acrylic acid alkyl ester, amonoethylenically unsaturated monomer copolymerizable with the(meth)acrylic acid alkyl ester is used to improve adhesive property,cohesive force, heat resistance, etc. Such a monoethylenicallyunsaturated monomer includes acrylic acid, itaconic acid, sulfopropylacrylate, hydroxyalkyl acrylate, cyanoalkyl acrylate, acrylamide,substituted acrylamide, vinylcaprolactam, acrylonitrile, 2-methoxyethylacrylate, glycyl acrylate, vinyl acetate, etc., and according to thepurpose, those are used alone or as a mixture of two or more kinds.

In the monomer of the component a), the use proportion of the(meth)acrylic acid alkyl ester and the monoethylenically unsaturatedmonomer copolymerizable therewith is that the former is from 70 to 100%by weight, and preferably from 85 to 95% by weight, and the latter isfrom 30 to 0% by weight, and preferably from 15 to 5% by weight. Byusing such a use proportion, the adhesive property, the cohesive force,etc., can be balanced well.

The polyfunctional (meth)acrylate as the crosslinking agent which is thecomponent b) used in the present invention functions to increase shearstrength of the adhesive. Examples thereof include trimethylolpropanetri(meth)acrylate, pentaerythritol tetra(meth)acrylate, 1,2-ethyleneglycol di(meth)acrylate, 1,6-hexanediol di(meth)acrylate, etc.

The amount of this polyfunctional (meth)acrylate used is from 0.02 to 5parts by weight, and preferably from 0.1 to 3 parts by weight, per 100parts by weight of the monomer as the component a). Within this range,where a bifunctional (meth)acrylate is used, the amount thereof can beincreased, and where a trifunctional or more (meth)acrylate is used, theamount thereof can be decreased. However, when the amount is less than0.02 part by weight, the degree of crosslinking after thephotopolymerization cannot sufficiently be increased and when the amountis larger than 5 parts by weight, there is the possibility of loweringthe adhesive force, etc., which are either undesirable.

The photopolymerization initiator as the component c) used in thepresent invention includes benzoin ethers such as benzoin methyl ether,benzoin isopropyl ether, etc; substituted benzoin ethers such as anizoinmethyl ether, etc; substituted acetophenones such as2,2-diethoxyacetophenone, 2,2-dimethoxy-2-phenoneacetophenone, etc.;substituted α-ketols such as 2-methyl-2-hydroxypropiophenone, etc.;aromatic sulfonyl chlorides such as 2-naphthalenesulfonyl chloride,etc.: photoactive oximes such as1-phenone-1,1-propanedione-2-(o-ethoxycarbonyl)oxime, etc.

The photopolymerization initiator is used in an amount of from 0.01 to 5parts by weight, and preferably from 0.05 to 1 part by weight, per 100parts by weight of the monomer as the component a). If the amountthereof is less than 0.01 part by weight, the monomer tends to remainlargely, while if the amount is more than 5 parts by weight, themolecular weight of the polymer is decreased and the cohesive force ofthe adhesive tends to decrease.

The flame retardant as the component d) used in the present inventionhas a melting point of at least 60° C., and preferably at least 80° C. Abromine-based flame retardant is particularly preferred. If a flameretardant having a melting point of lower than 60° C. is used, holdingcharacteristics of the photopolymerizable composition at hightemperature are decreased. Also, a chlorine-based flame retardant isundesirable because the composition generates a gas such as hydrogenchloride, etc., at high temperature, causing the possibility ofcorrosion of electronic parts.

Examples of the bromine-based flame retardant having a melting point ofat least 60° C. include propylene-modified tetrabromobisphenol A, acarbonyl-modified tetrabromobisphenol A oligomer, decabromodiphenylether, hexabromobenzene, penta-bromotoluene, hexabromocyclododecane,etc.

The flame retardant is used in an amount of from 10 to 180 parts byweight, preferably from 15 to 150 parts by weight, and more preferablyfrom 20 to 100 parts by weight, per 100 parts of the monomer as thecomponent a). If the amount of the flame retardant is less than 10 partsby weight, the flame retardance is decreased, and if the amount thereofis more than 180 parts by weight, the adhesive properties are decreased.In addition, conventional flame retardant assistants such as antimonytrioxide, etc., may be used together with the above-described flameretardant.

The photopolymerizable composition of the present invention comprisesthe above-described components a) to d) as essential components. Forpreparing the photopolymerizable composition, the monomer as thecomponent a) is mixed with the photopolymerization initiator as thecomponent c) and the premix is partially polymerized to form a coatablesyrup form having a viscosity of from about 50 to 5,000 centipoise.Further, the above-described premix of the monomer and thephotopolymerization initiator may be mixed with a thixotropic agent suchas fumed silica to form a coatable syrup form.

The syrup form mixture thus obtained is then mixed with thepolyfunctional (meth)acrylate as the crosslinking agent of the componentb), the flame retardant having a melting point of at least 60° C. as thecomponent d), and, if necessary, an additional photopolymerizationinitiator to prepare a photopolymerizable composition. This compositioncan further contain, if necessary, conventional additives such as afiller, an antioxidant, a tackifying resin, etc., in the range of notinhibiting the photopolymerization by irradiation with ultraviolet rays.

In the present invention, the photopolymerizable composition thusprepared is irradiated with ultraviolet rays to form a photopolymerizedproduct. The irradiation with ultraviolet rays is carried out in anoxygen-free atmosphere replaced with an inert gas such as nitrogen,etc., or in the state of being intercepted from air by covering with anultraviolet transmittable film. The ultraviolet rays are anelectromagnetic radiation having a wavelength range of from about 180 to460 nm. An electromagnetic radiation having a wavelength longer than orshorter than the above-described range. The ultraviolet source used is ageneral illuminator such as a mercury arc lamp, a carbon arc lamp, alow-pressure mercury lamp, an intermediate-pressure or high-pressuremercury lamp, a metal halide lamp, etc. The intensity of the ultravioletrays can appropriately be set up by controlling the distance to amaterial to be irradiated or by controlling the voltage. However, inview of the irradiation time (productivity), it is desirable to use aweak light having an intensity of from 0.1 to 7 mW/cm².

In the photopolymerized product after irradiation with ultraviolet rays,unreacted monomers usually remain in an amount of from about 1 to 5% byweight. The residual monomers are volatilized in using the adhesive athigh temperature to cause blister of the adhesive surface orcontamination with a gas, thereby causing decrease of the cohesiveforce. Therefore, it is generally desirable to remove the residualmonomers by heat-drying after the irradiation with ultraviolet rays. Theheat-drying is usually carried out using a drying furnace, but it isparticularly preferable to carry out the heat-drying using a hotblast-circulating drying furnace having a far infrared heater. Thereason for this is that drying can be conducted with good efficiency ina short period of time by heating from the inside with the far infraredheater and by the diffusion from the surface due to the circulating hotblast. The heat-drying treatment is generally conducted at a temperatureof from about 100 to 150° C. for several tens of second to severalminutes although varying depending on the capacity of the dryingfurnace.

The photopolymerized product thus obtained has a sufficiently highmolecular weight and also has a high degree of crosslinking such thatthe inside of the product is crosslinked with the crosslinking agent asthe component b) and the solvent-insoluble content is at least 50% byweight, and preferably from 70 to 95% by weight.

The present invention makes such a photopolymerized product into thepressure-sensitive flame-retardant adhesive. The adhesive has excellentadhesive force and holding power, in particular, has excellent holdingpower at high temperature and good heat resistance, and also has a highself fire-extinguishing property, that is, high flame retardance, due tothe above-described molecular weight and degree of crosslinking.

The adhesive sheets of the present invention are prepared by formingsuch a pressure-sensitive flame-retardant adhesive having excellent heatresistance on one surface or both surfaces of a substrate usually havinga thickness of about 25 to 125 μm such that the thickness of one layerof the adhesive is usually about 10 to 150 μm, thereby forming into atape form or a sheet form.

The substrate used is a porous material such as a non-woven fabric or apaper, and various kinds of plastics. In particular, for aheat-resistant use, heat-resistant films such as a polyamide film, apolyester film, polytetrafluoroethylene film, a polyether ketone film, apolyether sulfone film or polymethylpentene film; flame-retardant filmsor non-woven fabrics; and metal foils such as a copper foil or aluminumfoil are preferably used as a heat-resistant substrate.

Such adhesive sheets are produced, for example, as follows. A layer ofthe pressure-sensitive flame-retardant adhesive having excellent heatresistance is formed on a releasing liner, and the layer is transferredonto one surface or both surfaces of the substrate; alternatively, thesubstrate is directly coated or impregnated with the photopolymerizablecomposition without using the releasing liner. The layer thus formed isirradiated with ultraviolet rays to form a photopolymerized product, andthe layer is dried by heating to form the layer of thepressure-sensitive flame-retardant adhesive having excellent heatresistance. An appropriate method can be used depending on the kind ofthe substrate used.

BEST MODE FOR PRACTICING THE INVENTION

The present invention is described in more detail below by referring tothe following examples. In the examples and the comparative examples,all parts are by weight unless otherwise indicated.

EXAMPLE 1

A premix composed of 60 parts of isononyl acrylate, 28 parts of n-butylacrylate, 12 parts of acrylic acid, and 0.1 part of2,2-dimethoxy-2-phenylacetone (photopolymerization initiator) waspartially polymerized by exposing it to ultraviolet rays in a nitrogengas atmosphere to prepare a coatable syrup having a viscosity of 4,500centipoise. 0.2 Part of trimethylolpropane triacrylate (crosslinkingagent), 30 parts of propylene-modified tetrabromobisphenol A (FG-3100,trade name, made by Teijin Limited, melting point: 90 to 105° C.) (flameretardant) and 10 parts of antimony trioxide (flame retardantassistance) were added to 100 parts by the syrup to prepare aphotopolymerizable composition.

This photopolymerizable composition was coated on a substrate composedof a polyimide film having a thickness of 25 μm, and the layer coatedwas subjected to a photopolymerization treatment by irradiating withultraviolet rays from a high-pressure mercury lamp having an intensityof 5 mW/cm² at 900 mj/cm² in a nitrogen gas atmosphere. Thephotopolymerized layer was then dried at 130° C. for 5 minutes with ahot blast circulating dryer to form a layer of a pressure-sensitiveflame-retardant adhesive having a thickness of 50 μm. Thus an adhesivesheet was prepared.

EXAMPLE 2

A photopolymerizable composition was prepared in the same manner as inExample 1 except that a carbonyl-modified tetrabromobisphenol A oligomer(FG-8100, trade name, made by Teijin Limited, melting point: 165 to 180°C.) was used as the flame retardant. Further, using thisphotopolymerizable composition, an adhesive sheet was prepared in thesame manner as in Example 1.

EXAMPLE 3

A photopolymerizable composition was prepared in the same manner as inExample 1 except that a premix composed of 90 parts of 2-ethylhexylacrylate, 10 parts of acrylic acid, and 0.1 part of2,2-dimethoxy-2-phenylacetone (photopolymerization initiator) was used.Further, using this photopolymerizable composition, an adhesive sheetwas prepared in the same manner as in Example 1.

EXAMPLE 4

A photopolymerizable composition was prepared in the same manner as inExample 3 except that a carbonyl-modified tetrabromobisphenol A oligomer(FG-8100, trade name, made by Teijin Limited, melting point: 165 to 180°C.) was used as a flame retardant. Further, using thisphotopolymerizable composition, an adhesive sheet was prepared in thesame manner as in Example 1.

Comparative Example 1

A photopolymerizable composition was prepared in the same manner as inExample 1 except that 30 parts of the flame retardant(propylene-modified tetrabromobisphenol A) and 10 parts of the flameretardant assistant (antimony trioxide) were not used. Further, usingthis photopolymerizable composition, an adhesive sheet was prepared inthe manner as in Example 1.

Comparative Example 2

A photopolymerizable composition was prepared in the same manner as inExample 1 except that 50 parts of chlorinated paraffin (ENPARA 40, tradename, made by AJINOMOTO CO., INC., liquid at normal temperature (i.e.,melting point: lower than normal temperature)) was used as the flameretardant and 20 parts of antimony trioxide was used as the flameretardant assistant. Further, using this photopolymerizable composition,an adhesive sheet was prepared in the same manner as in Example 1.

Comparative Example 3

90 Parts of 2-ethylhexyl acrylate, 10 parts of acrylic acid, 210 partsof ethyl acetate, and 0.4 part of 2,2-azobis-isobutyronitrile werecharged with a flask. After sufficiently replacing the atmosphere in thesystem with a nitrogen gas, a solution polymerization was conducted byheating to a temperature of from 60 to 80° C. with stirring. An acrylicpolymer solution having a viscosity of 120 poises, a conversion of 99.2%by weight, and a solid content of 31.4% by weight was obtained.

2.0 Parts of a polyfunctional isocyanate compound (crosslinking agent),30 parts of propylene-modified tetrabromobisphenol A (FG-3100, tradename, made by Teijin Limited, melting point: 90 to 105° C.) (flameretardant) and 10 parts by antimony trioxide (flame retardant assistant)were added to 100 parts of the acrylic polymer solution to form apressure-sensitive flame-retardant adhesive. This adhesive was coated ona substrate composed of a polyimide film having a thickness of 25 μm.After drying the coated layer at 40° C. for 5 minutes in a hot blastdryer, the coated layer was further dried at 130° C. for 5 minutes toform a layer of the pressure-sensitive flame-retardant adhesive having athickness of 50 μm. Thus an adhesive sheet was prepared.

The adhesive sheets obtained in Examples 1 to 4 and Comparative Examples1 to 3 were examined for adhesive force, holding power (shear holdingpower), and flame retardance in the following manners. The measurementresults are shown in Table 1 below.

Adhesive Force

The adhesive sheet was cut into a width of 20 mm to prepare a testpiece. Using a stainless steel plate as an 20 adherent, 180 degreereleasing was performed according to JIS Z 1522, and the adhesive force(g/20 mm width) in this case was measured.

Holding Power

The adhesive sheet was cut into a width of 10 mm to prepare a testpiece. This test piece was adhered to an aluminum plate such that theadhered area was 20 mm×10 mm. The assembly was allowed to stand for 30minutes at 80° C. A load of 500 g was applied to the end of the testpiece, and the time of falling by slipping was measured. The mark (*) inTable 1 shows that in the measurement, the layer of thepressure-sensitive flame-retardant adhesive caused cohesive failure andfell down.

Flame Retardance Test

According to a thin material vertical combustion test method of UL-94,combustion time of the adhesive sheet was determined.

TABLE 1 Flame retardance Adhesive force Holding power (Flame retardanttime) (g/20 mm width) (80° C.) (sec) Example 1 950 200 or more 2 Example2 850 200 or more 5 Example 3 900 200 or more 2 Example 4 800 200 ormore 5 Comparative 1,010   200 or more 52  Example 1 Comparative 1,100  5* 10  Example 2 Comparative 650 5* 5 Example 3

As is apparent from the results shown in Table 1, it cam be seen that inExamples 1 to 4, pressure-sensitive flame-retardant adhesives which haveexcellent adhesive force and holding power and also have a flameretardance, and the adhesive sheets using the adhesives are obtained byirradiating the photopolymerizable compositions with ultraviolet rays,but in Comparative Examples 1 to 3, the samples obtained are poor in atleast one of the adhesive force, the holding power and theflame-retardance.

INDUSTRIAL APPLICABILITY

As described above, the present invention can provide apressure-sensitive flame-retardant adhesive and the adhesive sheetsthereof, which have excellent adhesive properties such as adhesiveforce, holding power, etc., and also have a self fire-extinguishingproperty which instantaneously extinguishes fire, by using a(meth)acrylic acid alkyl ester as the main component, compounding aphotopolymerization initiator, a specific crosslinking agent and aspecific flame retardant with the ester to prepare a photopolymerizablecomposition, and polymerizing and curing the composition by irradiatingit with ultraviolet rays. Those can suitably be used in the fieldrequiring a high flame-retardance and a strong adhesive property, suchas fixation of electronic parts, building materials, vehicles, aircraft,ships, etc.

What is claimed is:
 1. A photopolymerizable composition comprising a)100 parts by weight of a monomer (or the oligomer thereof) comprising 70to 100% by weight of a (meth)acrylic acid alkyl ester having on theaverage a carbon number of 2 to 14 in the alkyl group, and 30 to 0% byweight of a monoethylenically unsaturated monomer copolymerizable withthe (meth)acrylic acid alkyl ester; b) 0.02 to 5 parts by weight of apolyftunctional (meth)acrylate as a crosslinking agent; c) 0.01 to 5parts by weight of a polymerization initiator; and d) 10 to 180 parts ofa bromine-based flame retardant having a melting point of at least 60°C.
 2. A pressure-sensitive flame-retardant adhesive comprising aphotopolymerization product of a photopolymerization compositioncomprising a) 100 parts by weight of a monomer (or oligomer thereof)comprising 70 to 100% by weight of a (meth)acrylic acid alkyl esterhaving on the average a carbon number of 2 to 14 in the alkyl group, and30 to 0% by weight of a monoethylenically unsaturated monomercopolymerizable with the (meth)acrylic acid alkyl ester; b) 0.02 to 5parts by weight of a polyfunctional (meth)acrylate as a crosslinkingagent; c) 0.01 to 5 parts by weight of a polymerization initiator; andd) 10 to 180 parts of a bromine-based flame retardant having a meltingpoint of at least 60° C.
 3. The pressure-sensitive flame-retardantadhesive as claimed in claim 2, wherein the adhesive has asolvent-insoluble content of at least 50% by weight.
 4. An adhesivesheet comprising a substrate having formed on one surface or bothsurfaces thereof a layer comprising a pressure-sensitive flame-retardantadhesive comprising a photopolymerized product of a photopolymerizablecomposition comprising a) 100 parts by weight of a monomer (or oligomerthereof) comprising 70 to 100% by weight of a (meth)acrylic acid alkylester having on the average a carbon number of 2 to 14 in the alkylgroup, and 30 to 0% by weight of a monoethylenically unsaturated monomercopolymerizable with the (meth)acrylic acid alkyl ester; b) 0.02 to 5parts by weight of a polyftunctional (meth)acrylate as a crosslinkingagent; c) 0.01 to 5 parts by weight of a polymerization initiator; andd) 10 to 180 parts of a bromine-based flame retardant having a meltingpoint of at least 60° C.
 5. The adhesive sheet as claimed in claim 4,wherein the substrate is a heat-resistant substrate.
 6. The adhesivesheet as claimed in claim 4, wherein the adhesive has asolvent-insoluble content of at least 50% by weight.